Replaceable stylus for stereophonic phonograph pickup

ABSTRACT

A magnetic stereophonic phono cartridge having a replaceable stylus unit. The stylus unit comprises a support body including an armature of permeance material provided with a stylus arm with a stylus at the free end thereof, the armature being supported on the support body by means of a ring of elastic material; and further including an annular magnet being magnetized in an axial direction and coaxially surrounding the armature between the two ends thereof and being positioned close to the ring of elastic material.

United States Patent 1191 Madsen et al.

[ Apr. 15, 1975 REPLACEABLE STYLUS FOR STEREOPHONIC PHONOGRAPH PICKUP[75] Inventors: Henning Schmidt Madsen, Hillerod;

Robert Gudmandsen, Taastrup, both of Denmark [73] Assignee: FonofilmIndustri A/S, Copenhagen V, Denmark 22 Filed: Aug. 18, 1972 21 Appl.No.: 281,810

[52] US. Cl 170/l00.41 D; l79/100.41 M;

179/100.41 K [51] Int. Cl H04r 11/00 [58] Field of Search 179/100.41 K,100.41 Z,

179/100.4l S, 100.41 M, 100.41 D; 274/37 [56] References Cited UNITEDSTATES PATENTS Pritchard l79/l00.4l K

3,538,266 l1/1970 Cho l79/100.41 K 3,694,586 9/1972 Grado 179/100.41 Z3,700,829 lO/1972 Anneberg.... 179/100.41 Z 3,763,335 lO/l973 Morita179/lOO.41 M

Primary ExaminerTerrell W. Fears Assistant ExaminerAlan Faber Attorney,Agent, or Firml-lyman l-lurvitz [57] ABSTRACT A magnetic stereophonicphono cartridge having a replaceable stylus unit. The stylus unitcomprises a sup port body including an armature of permeance mate- 26Claims, 3 Drawing Figures l6 1 I N a REPLACEABLE STYLUS FOR STEREOPHONICPHONOGRAPI-I PICKUP BACKGROUND OF THE INVENTION 1. Field of theInvention This invention, like that of U.S. Pat. application Ser. No.885,220, filed Dec. 15, 1969, now U.S. Pat. No. 3,700,829 relates toelectromagnetic transducers for reproduction of stereophonic phonographrecords and particularly to replaceable stylus assemblies in which aportion of the magnetic circuit is incorporated into the replaceableassembly.

More particularly, the invention relates to such transducers having amagnetic structure defining an airgap and comprising four magnetic legseach surrounded by its respective coil and forming an airgap in which astylus carrying armature is pivotally mounted; the armature also carriespermanent magnetic means for producing a magnetic field which cooperateswith the magnetic legs to form a magnetic circuit loop. By arranging themagnetic legs in two pairs perpendicular to each other the signalsproduced by movements of the stylus can be decomposed into twocomponents'each corresponding to its respective recording track of astereophonic record.

2. Description of the Prior Art In the early pickup assemblies of thistype, the armature itself is constituted by a permanent magnet. However,the still increasing requirements as to reproduction of signals at theupper end of te tone frequency spectrum has led to the development ofpickups in which the effective stylus mass has been reduced by using apermanent magnet placed outside the oscillating stylus system forproducing the magnetizing force, so that the armature may be constitutedby a light tube of magnetically conducting material such as soft iron.

The known pickups of this kind have the feature in common with othermagnetic pickup assemblies that the armature is coupled magnetically inseries with the coil carrying legs, so that the signal-producing fluxchanges are produced by alterations of the overall reluctance of themagnetic circuit caused by movement of the armature.

SUMMARY OF THE INVENTION According to the present invention, a novelstructure is provided in which the armature is so placed that it forms amagnetic load coupled in parallel with the permanent magnet or magnets,the four coil carrying legs forming two magnetic conductors each havingboth ends engaging the same pole face of the permanent magnet or polefaces of the same polarity of the permanent magnet. In the rest positionof the armature, in which the structure is perfectly symmetrical, bothends of each magnetic conductor are at the same magnetic potential andhence no flux is flowing through the coil carrying legs. When thearmature is deflected, however, the magnetic load on the permanentmagnet portion opposite one end of a magnetic conductor is increasedwhile the load on the magnet portion opposite the other end of saidconductor is decreased resulting in the occurrence of a magneticpotential difference between the ends of the magnetic conductor wherebya flux is produced in the coil carrying legs.

An object of the invention is to provide an improved electromagnetictransducer in which the effective mass of the oscillating stylus systemis reduced by placing the permanent magnet or magnets outside saidsystem.

It is a further object of the invention to provide a transducer in whichthe magnetic structure including the armature is perfectly symmertrical.

A still further object of the invention is to provide a magneticstructure for use in a transducer in which the coil carrying legs can bemade very thin thus allowing the use of signal coils having lowresistance and capacity.

Another object is to provide a replaceable stylus assembly in which thepermanent magnet portion is incorporated into the replaceable assembly.

Another object of the invention is to provide an electromagnetictransducer which is easy and cheap to produce.

Still another object of the invention is to provide a magnetic stereopickup having improved sound reproduction characteristics, particularlyat the upper end of the tone frequency range.

BRIEF DESCRIPTION OF THE DRAWING Other objects and advantages willappear from the following' description when taken in conjunction withthe accompanying drawings, in which:

FIG. 1 is a longitudinal section of an embodiment of the pickupaccording to the invention;

FIG. 2 is a section along the line IIII in FIG. 1; and

FIG. 3 is a perspective view of the front end of the magnetic systemwith the stylus assembly partially removed.

DESCRIPTION OF THE PREFERRED EMBODIMENT The pickup shown in FIGS. 1-3comprises a housing 1 cast in a suitable plastics material. At one endthe housing has a recess 7 in which is mounted a contact member 2 withfour legs 3, of which only two are shown in FIG. 1. At the other end ofthe housing is provided a replaceable stylus assembly comprising a castplastics support body 4 which, by means of a projecting holow guide pin5 of non-circular cross-section, engages a recess 6 formed in thehousing 1 and having the same cross-section.

Between the bottom of the recess 7 and the front face of the housing 1is provided a through-going recess with v a rear portion 8 ofsubstantially square cross-section and a front portion 9 thecross-section of which, as indicated in FIG. 2, is formed as a middlecircle and four peripheral circles tangential to the middle circle andequally spaced along the periphery thereof.

The said through-going recess 8, 9 serves as accommodation for themagnetic system which comprises four cylindrical legs 10 of permalloy,the front portions of which are disposed in their respective cylindricaledge portions of the recess 9 and extend rearwardly through the recess8. The rear ends of the legs 10 are attached to and interconnectedmagnetically by a cross member II, which is likewise made of permalloy.About each leg 10 is disposed a coil 12 which is mounted in the recess8, and each pair of diametrically opposed coils are connected to oneanother and to two of the legs 3 by leads 13.

In the lower portion of the support body 4 of the stylus assembly isformed an inclined cylindrical, throughgoing channel 15 opening in theunder face of the body and forming an elongation of the recess 9 in thehousing 1. In this channel is secured a -tube 16 of a nonmagneticmaterial, for instance aluminum, which projects from the support body 4and, when the stylus assembly is fitted, enters the cylindrical centralportion of the recess 9. An annular permanent magnet 14, which ismagnetized in the axial direction and which consists of a ceramic,magnetically anisotropic material is coaxially mounted about theexterior portion of tube 16 adjacent the face of body 4. When thesupport body 4 is mounted in the housing 1, the magnet 14 is disposed inan enlarged section of the recess 9 at the front of the housing 1.

A tubular soft iron armature 17 is mounted within the tube 16 by meansof an annular, resilient pad 18, for instance of rubber or neoprene, tobe freely pivotable in all directions about a point disposed in oradjacent to the center of the pad. The armature 17 is slightly longerthan the annular magnet 14 and is disposed coaxially with andsymmetrically with respect to the magnet. To the armature is attachedone end of a tubular, thinwalled aluminum stylus are 19 extendingthrough the channel 15. The free end of the sytlus arm 19 projectingoutwardly of the support body 4 is flattened and supports a stylus 20.

On the under face of the housing 1 is mounted a magnetic screen 21 whichextends upwardly along the front face of the housing between the housingand the support body 4 of the stylus assembly.

In accordance with the present invention, the annular magnet is firmlyattached to the replaceable stylus unit. By means of this specialarrangement a number of advantages are obtained. First of all, it isobvious that different types of annular magnets could be used in thereplaceable stylus unit so that is will be possible to alter the pickupby replacing the stylus unit with another having a different type ofmagnet. This would not be possible by a construction where the magnet isfixed in the housing, such as in Ser. No. 885,220, of which thisinvention constitutes an improvement.

It should also be mentioned that the armature could consist not only ofa permeance material but also of a magnet having its poles positionedopposite the poles of the annular magnet. If such an embodiment is used,it is, of course, of greatest importance that during the production ofthe stylus unit the two magnets be positioned correctly relatively toone another. Further, it would completely spoil the armature magnet if,by replacement ofthe stylus unit, this armature magnet were removed fromits position within the annular magnet. it might also be mentioned thatin the novel contruction disclosed herein the annular magnet will have atendency to support the forces holding the stylus unit firmly in itsposition with the annular magnet abutting the free ends of the four rodsin the housing.

The operation of the magnetic pickup comprising the composite units ofthe support body and main housing is fully disclosed in theaforementioned U.S. Pat. application Ser. No. 885,220, the pertinentportions of which are incorporated herein by reference. Generally, theanisotropic ceramic annular magnet 14 shows great reluctance in alldirections perpendicular to the axial direction and is thereforemagnetically equivalent to two permanent rod magnets disposed at the endand in extension of their respective legs 10. Assuming the two magnetsto be of equal strength, the magnetic conductor formed by the legs andthe cross member 11 will have the same magnetic potential at its two endfaces in the neutral position of the armature 17, and no flux willtherefore flow through this conductor. But a relatively high flux willpass from the north pole of each magnet through the armature to thesouth pole of the magnet, and as the armature has a much lowerreluctance than the ceramic magnet 14 and the airgaps in actual practiceare very small, this flux may, with some justification, be described asa short circuit flux.

1f the armature as a result of the stylus movement oscillates upwards inthe plane of the paper it will approach the upper magnet and therebyreduce the reluctance in the short circuit flux path for said magnet andsimultaneously move away from the lower magnet, with the result that theshort circuit flux thereof meets increased reluctance. In other words,the upper short circuit flux will be increased and the lower shortcircuit flux will be reduced. On account of the relatively high inherentreluctance of the magnets this will cause the magnetic potential at thenorth pole of the upper magnet to be reduced, while the magneticpotential at the north pole of the lower magnet will be increased. Thusa magnetic potential difference is created between the two ends of themagnetic conductor 10, 11, 10 in response to the armature movement,which results in a flux in the conductor inducing in the coil windingsl2 electrical voltages which added togetner represent the sensed soundsignal.

Armature oscillations resulting from the other sound channel follow adirection perpendicular to the plane of the paper and are converted inthe same manner to electrical voltages in the second half of theelectromagnetic system disposed in this plane.

The magnetic conductor l0, ll, 10 leads only the effective signal fluxwhich changes direction in response to the armature oscillations, andthe legs 10 can therefore be designed very thin without risking that themagnetic point of operation deviates from the rectilinear part of themagnetization curve.

The stereo pickup described above and shown in FIGS. 1-3 of the drawingscan be changed into a mono pickup, merely by removing one pair of legs10 and rotating the remaining pair an angle of approximately 45 aboutthe common axis of the magnetic structure with respect to the stylusassembly.

In the embodiment described, the armature 17 is constituted by athin-walled tube of soft iron, by which the effective inertia of theoscillating stylus assembly is minimized. It is within the scope of theinvention, however, to replace the soft iron armature by a permanentmagnet which may be tubular or rod shaped and which is poled oppositelyto the fixed, outer magnet, so that it increases the short circuit flux.in this way, the sensitivity of the pickup can-be increased to somedegree.

It will be understood that the invention is not limited to theembodiments shown and described and that it will be possible within thescope of the invention to modify the structure in various respects. Thusfor example, the legs 10 may be square or rectangular in cross-sectioninstead of circular as also the armature may have other cross-sectionshapes.

What is claimed is:

1. An electromechanical transducer comprising a housing having a boreextending at one end obliquely into one surface thereof and areplaceable stylus unit adapted to be removably inserted into said bore,said stylus unit comprising an annular permanent magnet, a styluscarrying armature resiliently pivotally supported within said unit anddisposed coaxially with said annular magnet, plural rods of a highpermeance material disposed within said housing and spaced around theaxis of said bore, means including magnetizable material magneticallyinterconnecting the inner ends of said rods, and coils mounted on saidrods, thearrangemjent of said rods within said housing being such thattheir free end faces abut one pole face of the annular permanent magnetwhen said stylus unit is inserted into said housing. V

2. The combination according to claim 1, wherein said armature is ahollow a'nnular cylinder havin g an axial length greater than the axiallength of said permanent magnet.

3. The combination according to claim 2, wherein is provided a stylusarm for said stylus, said stylus arm having-one end mounted within saidhollow annular cylinder and having a stylus at its other end.

4. The combination according to claim 3, wherein said armature is apermanent magnet.

5. The combination according to claim 3, wherein said armature isfabricated of magnetically permeant material.

6. In an electromechanical transducer comprising a housing having at oneobliquely 'into one surface thereof, four rods ofa highpermeancematerial extending parallel to and equiangularly spaced aroundthe axis of said bore and having outer and inner ends, meansmagnetically interconnecting the inner ends of said rods, and four coilsmounted on said rods respectively between the other end of said bore andsaid inner ends, said coils being series connected in pairs eachcomprising oppositely disposed coils and connected to respectiveterminals, the provision of a replaceable stylus unit adapted to beinserted into the housing by sliding the unit in the direction of theaxis of said bore in the housing, said stylus unit comprising a hollowtube of nonmagnetic material, an armature of permeance material providedwith a stylus arm with a stylus at the free end thereof, said armaturebeing supported with respect to said hollow tube by means ofa ring ofelastic material, and an annular permanent magnet magnetized in theaxial direction mounted on and externally of said hollow tube, saidmagnet being positioned for abutting said outer ends of said four rodswhen said hollow tube is located in said housing.

7. The combination according to claim 6, wherein said armature is ahollow cylinder, and wherein said stylus unit includes a stylus supportrod having ne end supported internally of said hollow cylinder andhaving another end supporting a stylus.

8. The combination according to claim 7, wherein said armature is apermanent magnet.

9. The combination according to claim 7, wherein said armature is ahollow cylinder of magnetically permeant material.

10. A transducer, comprising a plastic body having terminals extendingtherefrom, a stylus bearing replaceable cartridge, said body including arecess, said cartridge having an extension mating into said recess toprovide a rigid mounting for said cartridge relative to said body, saidbody mounting four relatively spaced permanent rods having inner endsand outer ends, means magnetically shorting said outer ends, saidcartridge including a stylus assembly which includes an annularpermanent magnet having its ends of opposite polarilties, a magneticarmature passing through said annular permanent magnet, means mountingsaid armature for free pivoting with respect to said permanent magnet, astylus arm secured to said armature at one end of said sytlusarm, astylus secured to the other end of said stylus arm, said permanentmagnet being positioned to abut said inner ends when said cartridge isinserted into said body, and coils connected to said terminalsandrespectively wound about said rods.

11. The combination according to claim 10, wherein is provided a hollownon-magnetic tube, a resilient annulus securing said armature internallyspaced with respect to said tube, said permanent magnet being mounted onsaid tube exteriorly of said tube, and said resilient annulus enablingfree pivoting of said armature in any direction with respect to the axisof said armature and of said tube.

12. The combination according to claim 11, wherein said resilientannulus extends axially beyond said permanent magnet, and wherein saidarmature. extends through said resilient annulus in one direction andbeyond said permanent magnet in a direction opposite to said onedirection, wherebysaid permanent magnet is axially coextensive with onlypart of said armature.

13. The combination according to claim 10, wherein said recess extendson an acute angle to said terminals.

14. The combination according to claim 10, wherein said body. includes afurther recess for accepting said stylus mounting, said further recessand said first mentioned recess having parallel axes.

15. The combination according to claim 11, wherein said body includes afurther recess for accepting said stylus mounting, said further recessand said first mentioned recess having parallel axes.

16. The combination according to claim 12, wherein said body includes afurther recess for accepting said stylus mounting, said further recessand said first mentioned recess having parallel axes.

17. The combination according to claim 13, wherein said body includes afurther recess for accepting said stylus mounting, said further recessand said first mentioned recess having parallel axes.

18. A replaceable stylus assembly for a phonograph record reproducer,comprising a non-magnetic hollow cylinder, a cylindrical axially hollowpermanent magnet mounted externally on said hollow cylinder, said magnethaving oppositely polarized ends, a flexible annulus mounted interiorlyof said hollow cylinder, said flexible annulus being coaxial with saidmagnet and offset axially from said magnet, a hollow cylindricalpermeance armature mounted interiorly of said annulus, said atmatureextending axially beyond said magnet in two directions, a stylus armhaving one end secured within said hollow of said armature and havinganother end externally of said armature, and a stylus secured to saidanother end.

19. The combination according to claim 18, wherein said armature is apermanent magnet having its ends oppositely polarized.

20. The combination according to claim 18, wherein said armature isfabricated of magnetically permeant material.

21. An electromechanical transducer comprising a housing having a boreextending at one end obliquely from one surface thereof and areplaceable stylus unit adapted to be removably inserted into said bore,said stylus unit comprising an annular permanent magnet, a styluscarrying armature pivotally supported within said unit and disposedcoaxially with said annular magnet, four rods of a high permeancematerial disposed within said housing extending parallel to andequiangularly spaced around the axis of said bore and extending beyondthe other end thereof, a cross piece of magnetizable materialmagnetically interconnecting the inner ends of said rods and fixablyheld in the said housing, and four coils mounted on said rods at theinner ends thereof adjacent said cross-piece, said coils being seriesconnected in pairs each comprising oppositely disposed coils andconnected to respective terminals, the arrangement of said rods withinsaid housing being such that their free end faces will abut one poleface of the annular permanent magnet, when the said stylus unit isinserted into the housing with its axis coinciding with the axis of saidbore.

22. An electromechanical transducer as claimed in claim 21 in which thestylus unit comprises a connecting member extending parallel to the axisof the armature, and wherein the housing comprises a recess adapted toreceive the connecting member when mounting the stylus unit on thehousing.

23. An electromechanical transducer as claimed in claim 21 wherein thereplaceable stylus unit further comprises a tubular member of anon-magnetizable material, an annular member of an elastic material forpivotally supporting the stylus carrying armature with said tubularmember coaxially therewith, said annular permanent magnet surroundingsaid tubular member coaxially therewith and adjacent said annularsupporting member therein.

24. The combination according to claim 21, wherein said armature is apermanent magnet.

25. The combination accordng to claim 21, wherein said armature hasmagnetic permeance.

26. In an electromechanical transducer comprising a housing having abore extending at one end obliquely from one surface thereof, four rodsof a high permeance material extending parallel to and equiangularlyspaced around the axis of said bore and extending beyond the other endof said bore, a cross-piece of magnetizable material magneticallyinterconnecting the inner ends of said rods and fixedly held in saidhousing, and four coils mounted on said rods respectively between theother end of said bore and said cross-piece, said coils being seriesconnected in pairs each comprising oppositely disposed coils andconnected to respective terminals, the provision of a replaceable stylusunit adapted to be inserted into the housing by sliding the unit in thedirection of the axis of said bore in the housing, said stylus unitcomprising an armature of permeance material and being provided with astylus arm with a stylus at the free end thereof, said armature beingsupported in the unit by means of a ring of elastic material, and anannular permanent magnet being magnetized in the axial direction, saidmagnet being positioned for abutting the outer ends of said four rodswhen the stylus unit is attached to the housing.

1. An electromechanical transducer comprising a housing having a boreextending at one end obliquely into one surface thereof and areplaceable stylus unit adapted to be removably inserted into said bore,said stylus unit comprising an annular permanent magnet, a styluscarrying armature resiliently pivotally supported within said unit anddisposed coaxially with said annular magnet, plural rods of a highpermeance material disposed within said housing and spaced around theaxis of said bore, means including magnetizable material magneticallyinterconnecting the inner ends of said rods, and coils mounted on saidrods, the arrangement of said rods within said housing being such thattheir free end faces abut one pole face of the annular permanent magnetwhen said stylus unit is inserted into said housing.
 2. The combinationaccording to claim 1, wherein said armature is a hollow annular cylinderhaving an axial length greater than the axial length of said permanentmagnet.
 3. The combination according to claim 2, wherein is provided astylus arm for said stylus, said stylus arm having one end mountedwithin said hollow annular cylinder and having a stylus at its otherend.
 4. The combination according to claim 3, wherein said armature is apermanent magnet.
 5. The combination according to claim 3, wherein saidarmature is fabricated of magnetically permeant material.
 6. In anelectromechanical transducer comprising a housing having at oneobliquely into one surface thereof, four rods of a high permeancematerial extending parallel to and equiangularly spaced around the axisof said bore and having outer and inner ends, means magneticallyinterconnecting the inner ends of said rods, and four coils moUnted onsaid rods respectively between the other end of said bore and said innerends, said coils being series connected in pairs each comprisingoppositely disposed coils and connected to respective terminals, theprovision of a replaceable stylus unit adapted to be inserted into thehousing by sliding the unit in the direction of the axis of said bore inthe housing, said stylus unit comprising a hollow tube of non-magneticmaterial, an armature of permeance material provided with a stylus armwith a stylus at the free end thereof, said armature being supportedwith respect to said hollow tube by means of a ring of elastic material,and an annular permanent magnet magnetized in the axial directionmounted on and externally of said hollow tube, said magnet beingpositioned for abutting said outer ends of said four rods when saidhollow tube is located in said housing.
 7. The combination according toclaim 6, wherein said armature is a hollow cylinder, and wherein saidstylus unit includes a stylus support rod having ne end supportedinternally of said hollow cylinder and having another end supporting astylus.
 8. The combination according to claim 7, wherein said armatureis a permanent magnet.
 9. The combination according to claim 7, whereinsaid armature is a hollow cylinder of magnetically permeant material.10. A transducer, comprising a plastic body having terminals extendingtherefrom, a stylus bearing replaceable cartridge, said body including arecess, said cartridge having an extension mating into said recess toprovide a rigid mounting for said cartridge relative to said body, saidbody mounting four relatively spaced permanent rods having inner endsand outer ends, means magnetically shorting said outer ends, saidcartridge including a stylus assembly which includes an annularpermanent magnet having its ends of opposite polarilties, a magneticarmature passing through said annular permanent magnet, means mountingsaid armature for free pivoting with respect to said permanent magnet, astylus arm secured to said armature at one end of said sytlus arm, astylus secured to the other end of said stylus arm, said permanentmagnet being positioned to abut said inner ends when said cartridge isinserted into said body, and coils connected to said terminals andrespectively wound about said rods.
 11. The combination according toclaim 10, wherein is provided a hollow non-magnetic tube, a resilientannulus securing said armature internally spaced with respect to saidtube, said permanent magnet being mounted on said tube exteriorly ofsaid tube, and said resilient annulus enabling free pivoting of saidarmature in any direction with respect to the axis of said armature andof said tube.
 12. The combination according to claim 11, wherein saidresilient annulus extends axially beyond said permanent magnet, andwherein said armature extends through said resilient annulus in onedirection and beyond said permanent magnet in a direction opposite tosaid one direction, whereby said permanent magnet is axially coextensivewith only part of said armature.
 13. The combination according to claim10, wherein said recess extends on an acute angle to said terminals. 14.The combination according to claim 10, wherein said body includes afurther recess for accepting said stylus mounting, said further recessand said first mentioned recess having parallel axes.
 15. Thecombination according to claim 11, wherein said body includes a furtherrecess for accepting said stylus mounting, said further recess and saidfirst mentioned recess having parallel axes.
 16. The combinationaccording to claim 12, wherein said body includes a further recess foraccepting said stylus mounting, said further recess and said firstmentioned recess having parallel axes.
 17. The combination according toclaim 13, wherein said body includes a further recess for accepting saidstylus mounting, said further recess and said first mentioned recesshaving parallel axes.
 18. A replaceable stylus assembly for a phonographrecord reproducer, comprising a non-magnetic hollow cylinder, acylindrical axially hollow permanent magnet mounted externally on saidhollow cylinder, said magnet having oppositely polarized ends, aflexible annulus mounted interiorly of said hollow cylinder, saidflexible annulus being coaxial with said magnet and offset axially fromsaid magnet, a hollow cylindrical permeance armature mounted interiorlyof said annulus, said armature extending axially beyond said magnet intwo directions, a stylus arm having one end secured within said hollowof said armature and having another end externally of said armature, anda stylus secured to said another end.
 19. The combination according toclaim 18, wherein said armature is a permanent magnet having its endsoppositely polarized.
 20. The combination according to claim 18, whereinsaid armature is fabricated of magnetically permeant material.
 21. Anelectromechanical transducer comprising a housing having a boreextending at one end obliquely from one surface thereof and areplaceable stylus unit adapted to be removably inserted into said bore,said stylus unit comprising an annular permanent magnet, a styluscarrying armature pivotally supported within said unit and disposedcoaxially with said annular magnet, four rods of a high permeancematerial disposed within said housing extending parallel to andequiangularly spaced around the axis of said bore and extending beyondthe other end thereof, a cross piece of magnetizable materialmagnetically interconnecting the inner ends of said rods and fixablyheld in the said housing, and four coils mounted on said rods at theinner ends thereof adjacent said cross-piece, said coils being seriesconnected in pairs each comprising oppositely disposed coils andconnected to respective terminals, the arrangement of said rods withinsaid housing being such that their free end faces will abut one poleface of the annular permanent magnet, when the said stylus unit isinserted into the housing with its axis coinciding with the axis of saidbore.
 22. An electromechanical transducer as claimed in claim 21 inwhich the stylus unit comprises a connecting member extending parallelto the axis of the armature, and wherein the housing comprises a recessadapted to receive the connecting member when mounting the stylus uniton the housing.
 23. An electromechanical transducer as claimed in claim21 wherein the replaceable stylus unit further comprises a tubularmember of a non-magnetizable material, an annular member of an elasticmaterial for pivotally supporting the stylus carrying armature with saidtubular member coaxially therewith, said annular permanent magnetsurrounding said tubular member coaxially therewith and adjacent saidannular supporting member therein.
 24. The combination according toclaim 21, wherein said armature is a permanent magnet.
 25. Thecombination accordng to claim 21, wherein said armature has magneticpermeance.
 26. In an electromechanical transducer comprising a housinghaving a bore extending at one end obliquely from one surface thereof,four rods of a high permeance material extending parallel to andequiangularly spaced around the axis of said bore and extending beyondthe other end of said bore, a cross-piece of magnetizable materialmagnetically interconnecting the inner ends of said rods and fixedlyheld in said housing, and four coils mounted on said rods respectivelybetween the other end of said bore and said cross-piece, said coilsbeing series connected in pairs each comprising oppositely disposedcoils and connected to respective terminals, the provision of areplaceable stylus unit adapted to be inserted into the housing bysliding the unit in the direction of the axis of said bore in thehousing, said stylus unit comprising an armature of permeance materialand being provided with a stylus arm with a stylus at the free endthereof, said armature being supported in the Unit by means of a ring ofelastic material, and an annular permanent magnet being magnetized inthe axial direction, said magnet being positioned for abutting the outerends of said four rods when the stylus unit is attached to the housing.